Very long capillary columns (i.e., &gt;60 m) containing conventional stationary phases are useful for the separation of a large number of environmentally important compounds having different volatilities. Separation of various isomers of a compound, however, particularly isomers having similar volatilities, is very difficult using these capillary columns containing conventional stationary phases.
Liquid crystal compounds have shown particular selectivity and sensitivity as stationary phases for the separation of isomers having similar volatilities. Since the initial discovery of this property, research has been directed at the development of liquid crystalline compounds exhibiting broad liquid crystalline temperature ranges (i.e. broad phase transition temperature ranges), higher column operating temperatures, and lower bleed (i.e. loss of mobile phase).
Studies have also been undertaken to determine the correlation between the separation of positional isomers of a compound (e.g meta- and para-substituted benzene) and the structure of the liquid crystalline compounds used as the stationary phases. See K. Naikwadi et al., Can. J Chem., 65:970 (1987). These studies have shown that the separation factor for a particular pair of isomers is dependent on the chemical structure of the liquid crystalline compound used as stationary phase. This correlation between structure and separation provided very important clues for the development of specific liquid crystalline compounds to be used in chromatographic separation of isomers of a compound.
Although conventional stationary phases have proven unsatisfactory to date, the separation of isomers having similar volatilities can be achieved with capillary or packed column gas chromatography using stationary phases composed of liquid crystalline polymers (LCPs). For a LCP to be useful as the stationary phase in chromatography, the LCP must exhibit high thermal stability, broad liquid crystalline temperature range, and high upper transition temperature. Optimal performance depends on additional factors, such as the structure of the mesogenic group (i.e. the side chain of the LCP), the degree of polymerization, the packing of the side chains of the LCP, and the procedure used to prepare the columns.
Liquid crystal polymers known to be useful as stationary phases for chromatography are based on either a phenyl carboxylate ester or biphenyl carboxylate ester. The performance of these side chain liquid crystalline polysiloxanes (SCLCPs) as stationary phases, however, is not completely satisfactory. For example, known SCLCPs exhibit low column stability due to their volatility as well as longer retention times (and, concomitantly, lower efficiency) for polyaromatic hydrocarbon samples due to the low effective temperature limit of the columns. Thus, there is a need in the art for SCLCPs exhibiting a broad liquid crystalline temperature range for use in highly stable, efficient and selective capillary columns.
Moreover, the toxicity assessment of any environmental sample containing isomeric compounds requires separation of all the isomeric components thereof. For example, in polychlorinated dibenzo-p-dioxins, 2,3,7,8-tetrachlorodibenzo-p-dioxin (T.sub.4 CDD) is the most toxic isomer. The separation of 2,3,7,8-T.sub.4 CDD from the other 21 T.sub.4 CDD isomers is essential to determine the toxicity of a particular sample. Similarly, in the case of furans the separation of 2,3,7,8-tetrachlorodibenzofuran (2,3,7,8-T.sub.4 CDF) is required from the other 37 T.sub.4 CDF isomers. Limited success has been achieved using known polymeric liquid crystal columns (see, e.g., U.S. Pat. No. 4,909,935) to separate these various isomers. Thus, there is a need in the art for capillary columns useful in the separation of isomers of toxic compounds, particularly compounds such as PCDD and PCDF.